Prevalence of Acetabular Dysplasia at a Mean age of 18 Years After Treatment for Neonatal hip Instability

Background: The long-term radiological outcomes after the treatment of neonatal hip instability (NHI) in developmental dysplasia of the hip are unclear. Therefore, the prevalence of acetabular dysplasia at a mean age of 18 years after treatment was investigated. The relationship between acetabular dysplasia and hip discomfort has also been poorly established. Therefore, the differences in pain, hip-related quality of life (QOL), and hip impingement tests in hips with and without acetabular dysplasia were assessed. Methods: All 127 patients treated for NHI from 1995 to 2001 at the study hospital and meeting the inclusion criteria were invited to participate in this population-based follow-up. Of these individuals, 88 (69.3%) participated. The lateral center-edge angle (LCEA), Sharp’s angle (SA), and acetabular head index (AHI) were calculated for both hips from pelvic anterior–posterior radiographs. The Copenhagen Hip and Groin Outcome Score (HAGOS) questionnaire was completed for both hips separately; the total score, pain, and QOL subscores were calculated; and the impingement test was performed. Results: The prevalence of acetabular dysplasia, defined as an LCEA <20°, was 3.4%. Only the mean AHI (81.1%, SD 5.3) differed (-5.08, 95% CI -5.77 to -4.38, P <0.001) from previously described gender-specific and side-specific means, whereas the mean LCEA and SA did not. The odds ratio for a positive hip impingement test was 2.8 (95% CI: 1.11-7.05, P = 0.029) for hips with an LCEA <25° compared to hips with LCEA ≥25°. The hips with an SA ≥45° had a mean of 7.8 points lower for the HAGOS pain subscore (95% CI: 4.2-11.4, P <0.001) and a mean of 6.1 points lower for the HAGOS QOL subscore (95% CI: 2.1-10.2, P = 0.003) compared with hips with an SA <45°. Conclusions: The prevalence of acetabular dysplasia was low after treatment for NHI. Acetabular dysplasia seems to be as common in the general population as for those treated for NHI based on the LCEA. Only an SA ≥45° resulted in slightly more pain and lower hip-related QOL.

(J Pediatr Orthop 2024;44:e478-e484) N eonatal hip instability (NHI) is the term used to de- scribe when a newborn's hip is dislocatable or dislocated in clinical tests or ultrasound examination within the first month of life as part of the very early phase of the disease spectrum of developmental dysplasia of the hip (DDH).Treated NHI is associated with a 2.6 times greater risk for total hip replacement in young adulthood. 1Unfortunately, long-term radiological results after treatment for NHI are still lacking.
3][4][5] However, the risk of persisting acetabular dysplasia after treatment is well known, 3,4,6,7 and dysplasia might occur during growth. 8tudies with long follow-up periods have shown frequent acetabular dysplasia, but they have included large proportions of patients treated late. 3,8,9As late presentation is a risk factor for failure, 10 previous studies' results might not apply to patients who received treatment early.
The present study was the first population-based study to present long-term radiological results for treated NHI.This study investigated the prevalence of acetabular dysplasia, with Wiberg's lateral center-edge angle (LCEA) 11 as the primary outcome measure in 14-to 21year-old adolescents treated for NHI.Primarily, it was hypothesized that there is a difference in the LCEA in participants treated for NHI compared with those of a previously reported cohort of adolescents in a normal population. 12Secondarily, it was hypothesized that there is a difference in Sharp's acetabular angle (SA) and the acetabular head index (AHI) in participants treated for NHI compared with those of a previously reported cohort of adolescents in the normal population.Further, it was investigated if there was a difference in pain or hip-related quality of life (QOL) on the Copenhagen Hip and Groin Outcome Score (HAGOS) 13 or impingement tests 14 in hips with acetabular dysplasia.Previous studies have not evaluated hip-related QOL compared with hip morphology in those treated for DDH or NHI.

METHODS
This was a long-term follow-up study of previous patients born from 1995 to 2001 who received early treatment (at <1 month of age) for NHI at Oulu University Hospital.The hospital medical records were searched for the relevant ICD-10 and ICD-9 codes.Patients whose treatment began after one month, with a relevant concomitant disease that could influence hip development, who had a preterm birth, with teratogenic luxation, or who were living more than 300 km from the hospital were excluded.The study was population-based as the hospital was the only institution providing treatment for DDH in the North Ostrobothnia district.Invitations to participate were sent to 127 previous patients who were a mean of 18 years old, and 88 (69.3%) had radiographs suitable for analysis (Fig. 1).

Radiological Examination
The pelvis's standard supine anterior-posterior radiograph (AP) was recorded at the pediatric radiology unit.Images with the greater trochanter seen laterally and the lesser trochanter partially superimposed on the femoral neck were accepted.The AP rotational image quality was ensured by visual inspection of the obturator rings' symmetry, the iliac wings' concavity, and acetabular teardrops.The Tönnis Foramen Obturator Index (FOI) 15 was calculated, and images with FOI 0.7 to 1.8 were included for radiographic analysis. 16l image-based parameters were measured digitally by 1 author with HTML4/5-based software (neaLink, Neagen Ltd, Oulu, Finland).The author was proficient in the measurements by using measurements > 5 years in clinical practice.The most lateral point of the lateral rim was used in the measurements, and possible osteophytes were included.The inter-teardrop line was used as the transverse axis of the pelvis. 12

Outcome Measures
The LCEA, the primary outcome measure, is the most common measurement for acetabular dysplasia.It is formed by a vertical line and a line connecting the femoral head center with the lateral edge of the acetabulum (Fig. 2a). 118][19] An additional threshold of ≤ 25°was included for borderline dysplasia. 20

Secondary Outcome Measures
The SA describes the acetabulum's outlet angle formed between the acetabular roof's lateral margin and the inferior aspect of the pelvic teardrop and inter-teardrop line (Fig. 2b). 21Values ≥ 45°were considered indicative of dysplasia. 17,22he AHI quantifies the horizontal coverage of the femoral head by the acetabulum (Fig. 2c).It is calculated by dividing the width of the femoral head beneath the acetabulum by the horizontal diameter of the femoral head. 23Values <75% are indicative of dysplasia. 12,17,24he participants were asked to complete the HAGOS for both hips separately. 13HAGOS is considered the most accurate questionnaire for evaluating hip pain in young adults, as it was developed for physically active patients. 25he questions were rated on a Likert scale of 5 items.The individual scores were converted to produce the total score, the pain, and the QOL subscale summary scores between 0 and 100, where 0 represented extreme hip and groin problems, and 100 represented no hip and groin problems.
Physical examinations were performed by Mari Maikku.The examiner was blinded to hip complaints, radiographs, and the side of the prior NHI.The protocol was standardized and has been described before in a previous study. 26The impingement test (flexion, abduction, and internal rotation) was performed, with groin pain indicating a positive test. 14e Demographic Data and Medical Histories Were Recorded This study complied with the ethical principles outlined in the Declaration of Helsinki.The study was registered in the National Clinical Trials Registry (NCT02431871).The local ethics committee accepted the protocol.All participants provided informed consent.

Statistical Analysis
Summary measurements were presented as means with SDs unless otherwise stated.When using the results of both hips, a linear mixed model (LMM) or generalized linear mixed model (GLMM) was used for the analyses.The participant was set as a random effect in the LMM and GLMM due to dependent hips.When comparing with the The radiological measurements between affected and contralateral hips in unilateral NHI were compared using the paired samples t-test for continuous data and conditional logistic regression for binomial data.The mean differences with a 95% confidence interval (95% CI) were calculated for the LMM and t-test.Odds ratios (ORs) with a 95% CI were used as the result of the GLMM and conditional logistic regression.
Two-tailed P-values were reported, and SAS (version 9.4, SAS Institute Inc., Cary, NC, USA) and SPSS for Windows (IBM SPSS Statistics for Windows, version 28.0, 2016 release, IBM Corp., Armonk, NY, USA) were used for the analyses.The checklist of items for cohort studies (Strengthening the Reporting of Observational Studies in Epidemiology statement; http://www.strobe-statement.org) was followed when reporting the data.

RESULTS
The mean age of the 88 participants (69 female, 19 male) was 18.2 years (SD 2.1, range 14-22).Neonatal treatment in von Rosen type splint (Coxa-splint, Pedihealth, Finland) had been started from a mean age of 0.54 weeks (SD 0.70).The mean treatment duration was 8.8 weeks (SD 2.6).There were altogether 118 hips with NHI, including 41 hips positive for the Ortolani test and 56 hips positive for the Barlow test.In 21 hips in 15 patients, the treatment was started based on clinical instability at the maternity ward, and an additional ultrasound examination was performed at a mean of 1.3 weeks of age (SD 1.1).Of those, 11 hips were Barlow positive, 4 were Ortolani positive, and 6 had suspicious findings but no apparent instability at the maternity ward.
In 1 patient, treatment failed, resulting in a spica cast treatment.After treatment, the patients had routine followups at a mean age of 5.9 months (SD 0.63) and 13.5 months (SD 2.2).The surveillance ended at the second follow-up in 75 patients (85.2%).Surveillance was transferred to another hospital for three participants.Among 11 participants, the follow-ups continued at the study hospital until a mean age of 1.9 years (SD 0.95, range 1.2 to 4.2).Of those, 4 individuals still had slight acetabular dysplasia or lateralization of the femoral head present in 7 hips.One had a suspicion of avascular necrosis of the femoral head.No one had required a later surgical procedure.
The prevalence of an LCEA <20°was 3.4% (3 patients, all unilaterally).All 3 hips with an LCEA <20°also had an SA ≥ 45°and an overall dysplastic appearance in the radiograph taken for this study, but they all had a normal acetabular index (AI) and AHI at 1 year of age.The prevalence of an SA ≥ 45°and AHI <75% were 19.3% and 14.8%, respectively.1.
The mean LCEA and SA did not differ significantly from sex-and side-specific means in the Bergen Birth Cohort (Table 2).However, the mean sex-adjusted and side-adjusted AHI was 5.08 lower for the present study (P < 0.001).
As dysplasia was rare, a small subgroup analysis was performed for clinical interest.There were 58 patients treated for unilateral NHI.The measurements of acetabular dysplasia did not differ significantly between affected and contralateral hips in unilateral NHI (Table 3).This was also true for Ortolani-positive hips and their contralateral stable hips.
The mean HAGOS total score was lower for hips with an LCEA <25°with a mean difference of 2.8 (P = 0.040, Table 4).The hips with an SA ≥ 45°had a significantly lower HAGOS pain subscore with a mean difference of 7.8 (P < 0.001) and lower HAGOS QOL subscore with a mean difference of 6.1 (P = 0.003).Additionally, the OR for the positive impingement test was 2.8 for hips with an LCEA <25°(P = 0.029, Table 4).

DISCUSSION
This population-based long-term follow-up study investigated the prevalence of acetabular dysplasia after the treatment of NHI with LCEA as the primary outcome measure.Only 3.4% of participants had an LCEA <20°.The sex-specific and side-specific mean values of LCEA did not show a significant difference between a previously described similar-aged cohort. 12Based on the LCEA, treated NHI did not predispose patients to acetabular dysplasia in 18 years of follow-up, and the primary hypothesis can be refuted.The prevalence of an SA ≥ 45°a nd AHI <75°were higher.The sex-specific and sidespecific mean values of the AHI compared to previously described similar-aged cohort 12 showed a significant difference.Partly, the secondary hypothesis can also be refuted.
Several studies have investigated the LCEA reference range based on the 2.5th percentile, and the lower limit of ~< 20°is widely recognized. 12,17,19The 3.4% prevalence of an LCEA <20°in this study was similar to the Bergen Birth Cohort (3.3%) 22 and a Danish population-based cohort (3.4%). 17Moreover, the present sexspecific and side-specific mean LCEA values are comparable to those for previous population-based European cohort studies. 17,27It has been speculated that dysplasia is more common in Asia than in Europe. 18The overall prevalence of LCEA <20°was 13.9% 18 and 15.0% 28 in cohorts of Japanese volunteers and asymptomatic South Korean adults, respectively.Also, Japanese adolescents had slightly lower mean values of LCEA (27.5°) than in the present study. 29Consequently, the present results of the low prevalence of acetabular dysplasia after NHI apply to Europe.Previous studies have suggested a tendency in patients treated for DDH to develop dysplasia during growth. 4,7he two long-term follow-ups of DDH until skeletal maturity had participants treated from a mean age of 4 months and found acetabular dysplasia was common. 8,9The present results emphasize the excellent prognosis of NHI in contrast to DDH with later treatment.Also, in the subgroup analysis, even the hips positive for the Ortolani test did not have more dysplastic measures compared with the nonaffected hips in unilateral NHI.
The studies with follow-ups until school age in patients with early treatment of DDH support the present results.Patients with successful early treatment had a similar mean LCEA and SA to those in the present study at a mean age of 10.2 years. 5Cashman et al found a mean AI value 4°larger than the values of the normal population at a mean surveillance of 6.5 years, but the prevalence of an LCEA <20°was only 3.5%, similar to the findings in the present study. 4he high prevalence of an SA ≥ 45°and AHI < 75°in the present study were interesting.Also, the mean AHI was significantly more dysplastic in the present study than in the Bergen Birth Cohort.Two recent Japanese studies reported a similar mean value for the AHI compared with that found in the present study. 18,29Nishimura et al reported an even higher mean SA and speculated whether the propensity for dysplastic values was caused by the young age (12 to 18 years) of the participants in their cohort. 29The clinical significance of these findings is unknown as the most important measurement for acetabular dysplasia, LCEA, did not indicate a higher rate of dysplasia.
In the present study, the mean difference for the HAGOS total score was only 2.8 points lower for hips with an LCEA ≤ 25°compared with hips with LCEA > 25°, which does not meet clinical relevance.However, an LCEA ≤ 25°predicted a positive hip impingement test with an OR of 2.8.The hips with an SA ≥ 45°had a mean 7.8 points lower pain subscore and a mean 6.1 points lower QOL subscore compared with hips with an SA < 45°, which met the smallest detectable change value at the group level reported by the questionnaire developers. 13he reference ranges for the LCEA, SA, and AHI are based on an arbitrary SD of 2 or percentiles of 2.5 or 5, not clinical relevance.To our knowledge, this is the first study to report that a high SA could be associated with hip complaints in a cohort setting.Therefore, further investigation should determine whether the SA is more predictive of hip complaints than the LCEA.In a previous study, there was no significant difference in the HAGOS scores for patients treated for NHI and age-matched and sex-matched controls. 26There are few reports on the association of acetabular dysplasia with hip complaints in the general population.In Japanese older adults, a low LCEA was associated with hip pain, but only 1.3% of hips with an LCEA of 20°to 25°had pain within the past month. 18Contrary to our findings, Engesaeter et al found no association between an LCEA <20°, SA ≥ 45°, or multiple radiological signs of hip dysplasia and The Western Ontario and McMaster Universities Osteoarthritis Index. 22

Strengths
The present study had some considerable strengths.First, the neonatal treatment was very similar in all participants.The setting was population-based as all the DDH patients in the district were treated at the same hospital, to which other hospitals referred suspected cases of DDH.The health care in Finland is based on a universal, publicly funded system that enables the centralized treatment of specific diseases.The participation rate (69.3%) was very high, especially considering the vast recruitment area and the extensive 18-year follow-up period.It was important to achieve high participation.If only those with complaints would participate, the prevalence of dysplasia could be false.The high rate was based on a few key things: (1) universal health care enables mainly up-todate contact information, and the citizens are known to be "obedient."(2) The participants were promised a voucher worth ~20 dollars as a small compensation for their effort.
(3) We were persistent in contacting every patient with up to 3 letters and 2 phone calls, and patients received several options on how to contact us back.(4) The patients had the imaging and physical examinations on the same day, and the questionnaires were returned before that, so all returned the forms.

Limitations
This study had limitations.The main limitation was the applicability of the present results to currently treated patients.Nearly all the participants had started treatment before 1 week old and based on clinical examination, and therefore, patients had the potential for spontaneous resolution.Barlow himself reported that as much as 88% of hips with positive Barlow test might stabilize in 2 months. 30The natural history of Ortolani-positive hips is less studied.However, the American Academy of Orthopaedic Surgeons Clinical Practice Guideline suggests immediate or delayed (2 to 9 weeks of age) treatment of Barlow and Ortolani-positive hips. 31Our institution's standard care pathway for DDH endorses an ultrasound examination at the age of 2-3 weeks without treatment being started for all infants with NHI.Subsequently, some of the treated patients would not receive treatment anymore.The treatment rate at that time at our institution was 0.7% 32 and is currently 0.29%. 33Thus, interpreting these results should be done with caution and cannot be extended to the long-term sequelae of today's treatment for DDH patients.This might also influence the overall low rate of dysplasia.
Fifteen patients received additional neonatal ultrasounds before treatment at a very early age.Ultrasound before six weeks of age has been held unreliable. 34However, it should be noted that only 6 of 118 affected hips, including those treated after ultrasound, had negative Ortolani-Barlow test at the maternity ward.
Moreover, the Bergen Birth Cohort does not necessarily represent the young population in Finland.However, it had the same mean age of the participants and an excellent study protocol and reporting.Moreover, several studies have already demonstrated the reproducibility of LCEA, SA, and AHI. 17,18,29We did not control for femoro-acetabular impingement or other hip pathology in the analysis or lifestyle factors, such as smoking, which leaves the possibility of residual confounding.Finally, as acetabular dysplasia was uncommon, the sample size for evaluating the association between hip-related QOL and radiographic parameters was too small to draw definite conclusions.
In this population-based long-term follow-up, participants treated for NHI were not predisposed to acetabular dysplasia, defined by an LCEA <20°.The relationship between radiological hip dysplasia and hip discomfort should be further investigated in a large population-based cohort.

FIGURE 1 .
FIGURE 1. Flowchart of eligible radiographs and exclusion criteria for participants.HAGOS indicates Hip and Groin Outcome Score.

TABLE 1 .
Proportions of Hips with Measures Indicating Borderline Dysplasia or Dysplasia in Both Hips by Sex and Hip Side all 176 hips in 88 participants, 149 (84.7%) had no radiographic findings indicating dysplasia; however, 19 hips (10.8%) had one measurement showing dysplasia, 6 hips (3.4%) had 2 measurements, and only 2 (1.1%) had all 3 measurements indicating dysplasia.The proportions of hips with borderline dysplasia or dysplasia are presented in Table AHI indicates acetabular head index; LCEA, lateral center-edge angle; SA, Sharp's acetabular angle.Of

TABLE 2 .
The Mean Values and SDs of Dysplasia Measurements in Comparison with the 17 to 20-year-old Population in the Bergen Birth Cohort Study.Comparisons were Made Considering Sex-specific and Side-specific Values AHI indicates acetabular head index; Diff, difference in means; LCEA, lateral center-edge angle; SA, Sharp's acetabular angle.

TABLE 3 .
The Differences Between Affected and Contralateral Hips in all Unilateral NHI and Unilateral Ortolani-positive NHI

TABLE 4 .
The Mean Differences with a 95% CI in an LMM for the HAGOS Total Score, Pain Subscore, and QOL Subscore, as well as the OR in a GLMM for Positive Impingement Tests and for Measurements Indicating Dysplastic and Nondysplastic Hips indicates confidence interval; FHEI, femoral head extrusion index; GLMM, generalized linear mixed model; HAGOS, The Copenhagen Hip Outcome Score; LCEA, Lateral center-edge angle; LMM, linear mixed model; OR, odds ratio; QOL, quality of life; ref, Reference group; SA, Sharp's acetabular angle CI